1. Field of the Invention
This invention relates to apparatus for drilling the curved portion of a subterranean borehole. More particularly, the invention relates to apparatus capable of initiating and controlling curved boreholes when drilling an oil or gas well.
2. Setting of the Invention
In producing subterranean fluids, such as oil and gas, thousands of feet of substantially vertical well bore or borehole are drilled into the earth to make a relatively few feet of contact with the producing formation or strata, since the formation often has limited vertical depth (some formations have as little as five feet) and much greater horizontal extent (e.g., thousands of feet). It is often desirable to increase the contact area of the borehole with the formation to increase the production rate of the well. Hydraulic fracturing is one known method of increasing the contact area which has been proven to increase production. Hydraulic fracturing is difficult to control.
Another known method of increasing the contact area of the borehole with the formation, which is the subject of this invention, is to drill horizontally or laterally into and through the formation. In order to do so, it is necessary to be able to drill a curved borehole, or curved section of borehole, from a substantially straight (normally vertical) borehole to the desired trajectory. Because of the limited vertical depth of some formations, in order to hit the formation with the curved borehole, it is desirable to be able to drill a curved borehole having a predictable radius of curvature. Even though it has the potential of being the least expensive and most reliable method of enhancing production rates by increasing the contact area of the borehole with the formation, curve drilling is not widely used at the present time because of shortcomings in the known curve drilling assemblies, such as in their durability, in their maintenance and operational expenses, and in their ability to drill a curved borehole having a predictable radius of curvature.
In order to accurately, repeatably, and predictably drill a curved subterranean borehole having a predetermined radius of curvature it is known that it is necessary to (1) initiate and maintain a deflection of the drill bit axis with respect to the axis of the borehole and (2) to control the azimuthal direction of the deflection in the borehole. Prior curve drilling assemblies have created and maintained the deflection by creating and maintaining a lateral force near the drill bit which forces the drill bit against the borehole sidewall in the desired azimuthal direction, and have controlled the azimuthal direction of the deflection by using a collar which engages the borehole sidewall or by using the weight of the drill string (weight-on-bit) to hold the deflection-creating device in position. The azimuthal direction is controlled so that the radius of curvature of the curved borehole will exist in a single radial plane, e.g., so the borehole will not have a helical or cork-screw trajectory and will drill in a single compass direction with respect to the borehole axis.
For example, U.S. Pat. Nos. 2,712,434; 2,730,328; 2,745,635; 2,819,040; 2,919,897; and 4,699,224 disclose using a collar on the drill string to create a lateral force which forces the drill bit into the borehole sidewall. U.S. Pat. Nos. 2,712,434 and 4,699,224 use a collar having an eccentric bore with the drill string passing through the eccentric bore to create the lateral force and the deflection. U.S. Pat. Nos. 2,730,328 and 2,745,635 effectively create a collar having an eccentric bore by extending shoes or springs from the collar when the collar is subjected to weight-on-bit so that the collar and drill bit are forced to one side of the borehole with the drill bit being forced into engagement with the borehole sidewall U.S. Pat. No. 2,819,040 uses a deflecting wedge which extends from the collar when the collar is subjected to weight-on-bit in order to create a lateral force which forces the drill bit into engagement with the borehole sidewall. U.S. Pat. No. 2,919,897 discloses using an eccentrically-shaped mandrel rotatable within an eccentrically-shaped deflection bearing to create a lateral force which forces the drill bit to one side of the borehole.
Once the collar is positioned to force the drill bit in the desired azimuthal direction, sidewall engaging ribs (U.S. Pat. Nos. 2,730,328 and 2,745,635), splines (U.S. Pat. No. 2,919,897), or angled projections or blades (U.S. Pat. Nos. 2,819,040 and 4,699,224) are used to prevent rotation of the collar with the drill string in order to fix the position, or rotational orientation, of the collar in the borehole and thereby fix the azimuthal direction of the deflection and the drill bit in the borehole. In U.S. Pat. Nos. 2,819,040 and 4,699,224 the projections or blades are angled to allow the collar to rotate with the drill string in one direction and to prevent rotation of the collar with the drill string in the other direction.
The above-mentioned patents also disclose apparatus for engaging the collar to the drill string for rotation with the drill string in order to rotationally orient the collar and thereby select the azimuthal direction of the deflection in the borehole. For example, U.S. Pat. Nos. 2,730,328 and 2,745,635 use the absence of weight-on-bit or axial loading to engage the collar to the drill string and the presence of weight-on-bit to disengage the collar from the drill string; U.S. Pat. No. 2,819,040 uses a hydraulically actuated piston; and U.S. Pat. Nos. 2,712,434 and 4,699,224 use a spring-actuated detent or cog which acts as a one-way clutch mechanism. U.S. Pat. No. 2,919,897 uses a key on the collar which engages a keyway in the drill string when the drill string is lifted and which disengages when weight is placed on bit and the splines on the collar have engaged the borehole to lift the collar with respect to the drill string and drill bit.
U.S. Pat. Nos. 2,687,282; 3,156,310; 3,398,804; 4,523,652; and 4,699,224 disclose the use of a flexible joint, such as a knuckle joint, to increase the magnitude of the deflection in order to drill a curved borehole having a shorter radius of curvature. U.S. Pat. Nos. 2,687,282; 3,398,804; and 4,523,652 disclose using a stationary deflecting surface in the vertical borehole to initiate and azimuthally direct the deflection of the drill bit and using the deflecting surface and weight-on-bit to control or maintain the desired azimuthal direction.
U.S. Pat. Nos. 3,156,310 and 4,699,224 use a collar in combination with the knuckle joint to initiate the deflection and to control or maintain the azimuthal direction of the deflection in the borehole. In U.S. Pat. No. 3,156,310 blades extend from the collar to deflect the collar and knuckle joint to one side of the borehole and to thereby deflect the drill bit against the opposite wall. The blades also serve to engage the borehole sidewall to prevent rotation of the collar and to maintain the azimuthal direction of the deflection. The collar may be on either side of the knuckle joint. U.S. Pat. No. 4,699,224 discloses angled blades extending from the thick side of an eccentrically bored collar. As in U.S. Pat. No. 3,156,310, the blades both deflect the collar to one side of the borehole and engage the sidewall In U.S. Pat. No. 4,699,224 the collar is on the uphole side of the knuckle joint.
In U.S. Pat. Nos. 2,919,897; 3,156,310; and 4,699,224 the weight of the drill string is carried in the non-rotating collar during drilling which creates a single wear-point. The deflection creates a radial component of the weight-on-bit which is directed towards and supported by the portions of the collar on the outside radius of the deflection and of the borehole.
U.S. Pat. Nos. 2,687,282 and 4,699,224 disclose forcing the drill bit to drill upward by leveraging the drill bit into the borehole sidewall using a reamer or stabilizer as a fulcrum between the drill bit and the knuckle joint U.S. Pat. Nos. 3,398,804 and 4,449,595 use a reamer, and U.S. Pat. No. 3,156,310 uses stabilizer blades, respectively, to leverage the drill bit into the borehole sidewall. This leveraging of the drill bit creates a lateral or radial force on the drill bit and also allows cutting forces to be leveraged from the drill bit into the drill string.
U.S. Pat. Nos. 2,687,282; 3,398,804; 4,449,595; 4,523,652; and 4,699,224 disclose using a reamer to ream the sidewall of the borehole. U.S. Pat. No. 4,523,652 discloses a curve drilling assembly in which the sidewall of a reamer is shaped to match the desired radius of curvature of a borehole in attempting to stabilize the downhole assembly when drilling into intervals of varying hardness U.S. Pat. No. 4,449,595 discloses a curve drilling assembly in which the reamer is designed to be overgauged, tapered, and non-cutting at its leading, downhole end in attempting to stabilize the reamer and prevent preferential upward cutting by the reamer. It is known that use of a reamer will both enlarge or "overgauge" the borehole diameter with respect to the drill bit diameter and will create lateral forces on the curve drilling assembly. Use of the reamer as a fulcrum will increase the overgauging of the borehole by the reamer because of the lateral forces exerted on the reamer.
U.S. Pat. No. 2,919,897 discloses biasing a selected "master" cutter on the drill bit into engagement with the borehole wall in the direction of the desired deflection and out of engagement with the borehole wall when the master cutter is oriented diametrically opposite to the direction of the desired deflection. U.S. Pat. No. 2,919,897 does not disclose or suggest controlling or modifying the gauge cutting of the remaining cutters on the drill bit.
U.S. Pat. No. 4,815,342, which is owned by the assignee of the this application, discloses a method of modeling the cutting surfaces on a drill bit, calculating the forces acting on the cutting surfaces, and calculating the position of balancing cutters which may be placed on the drill bit in order to reduce the imbalance force created by the cutters.
U.S. Pat. Nos. 5,010,789 and 5,042,596, which are owned by the assignee of the this application, disclose a drill bit and method of making a drill bit having a plurality of cutting elements (also referred to as "cutters") and a relatively smooth bearing zone. The cutting elements are positioned to cause the net imbalance force generated by the cutting elements to be directed towards the bearing zone in order to prevent backward whirl of the drill bit during drilling. Backward whirl results in severe impact loading of the cutters on the drill bit and is normally very detrimental to drill bits. Backward whirl is a motion that results in the longitudinal drill bit center moving counterclockwise around the borehole axis during drilling (the normal drilling direction being clockwise). In U.S. Pat. No. 5,042,596 the net radial imbalance force is disclosed as being created along a net radial imbalance force vector and as having sufficient magnitude to substantially maintain a sliding surface disposed in a cutter devoid region on the gauge portion of the drill bit in contact with the borehole wall.
Despite the many prior attempts to create a reliable curve drilling assembly, a need exists for a curve drilling assembly which will drill a curved borehole having a more reliable and predictable radius of curvature. The patents referenced in this application illustrate the long-felt need for a curve drilling assembly which will drill a curved borehole having these properties. There is also a commercial need for a curve drilling assembly which will drill a curved borehole with minimal maintenance and which is relatively inexpensive and easy to use.